Nutritional evaluation of corn and sorghum crop residues

Crop residue samples from 22 grazed and nongrazed corn and sorghum residue fields across Kansas were collected every 2 weeks during the 1997/98 winter feeding season to evaluate quality and yield. Corn residue averaged about 2 1⁄2 tons of total dry matter (DM)/acre, with the leaf fraction (attached and unattached) constituting about 60% of that residue. Unattached leaves represented 85% of the total leaf DM. Yield estimates for sorghum residue averaged over 2 tons/ acre. The stem component represented 55 to 60% of the total DM collected, and about 58% of the leaves remained attached to the stem.


Introduction
Tillage and fertilization practices and crop varieties have changed dramatically since the 1970's when the vast majority of crop residue research was conducted by Midwest universities.Moreover, today's harvesting equipment is much more efficient, so less grain is left in the field.The objective of this demonstration project was to obtain yield and quality estimates of corn and sorghum residue from across Kansas.

Experimental Procedures
Twenty-one County Extension Agents from across the state each identified a progressive livestock producer willing to participate in the study.At the onset of the grazing season, approximately ½ acre at each location was excluded from active grazing to evaluate the effects of weathering on residue nutrient content.
Residue was collected at 2-week intervals throughout the grazing season.For each sampling period, the residue in the grazed and nongrazed areas was collected from four different areas of the residue field in 8-foot row lengths and separated into attached leaves (manually removed from stem), unattached leaves, and stems (harvested above the brace root).After each sampling period, the collected samples were allowed air dry.All subsamples from each plant part and sampling period were weighed individually to estimate lbs DM/ acre and ground through a lawn chipper.The replicated samples were composited and sent to a commercial forage testing laboratory for analysis of DM and crude protein (CP) content.In addition, neutral detergent fiber (NDF) and acid detergent fiber (ADF) levels were determined, and the ADF concentrations were used to estimate TDN (see footnote in tables).To arrive at district averages, data from individual counties were composited by crop.Only counties with multiple sampling periods were included in this compilation.Means and standard deviations are presented for each residue type by plant part, as well as by grazed versus nongrazed.

Results and Discussion
Statewide averages for DM yield/acre and nutrient content of corn and sorghum residues are shown in T ables 1 and 4, respectively.Corn residue in the west averaged over 4 tons total DM/acre (Table 2).This value was almost double that from the central area and considerably higher than that from northeast Kansas.Corn leaves (attached and unattached) constituted 50 to 60% of residue DM.Unattached leaves represented almost 85% of the total leaf DM weight and 50% of the total plant DM collected.These observations are consistent with earlier reports citing the vulnerability of corn residue to wastage from trampling and snow.Total CP content of corn residue averaged about 5% and ranged from 4.60 to 5.60% across the state (Table 5).CP contents averaged about 5% in the leaf fraction and close to 4% in the stem fraction.The TDN content of the leaf fractions (unattached and attached) ranged from 48 to 51% throughout the state, whereas that of stems was close to 44%.
The DM yields of sorghum residue for central Kansas averaged about 2.0 tons per acre and is consistent with previous research reporting lower residue production per acre for sorghum than corn (Table 3).The stem component represented 55 to 60% of the total DM collected from the grazed and ungrazed locations.Stem percentage (as a percent of total DM) increased and leaf fractions declined as residue grazing progressed.In contrast to corn, about 58% of the sorghum leaves were attached.